1. Technical Field
The present invention relates in general to a voltage controlled oscillator and, in particular, to a voltage controlled oscillator for generating a desired frequency toward modulating a transmission signal on the generated frequency.
2. Background Art
Voltage controlled oscillators which additionally modulate a transmission signal are well-known in the art. These devices utilize a destablized transistor, either bipolar junction or field effect, to produce a modulated signal. A modulating signal is introduced across a first varactor--a device which exhibits voltage variable capacitance--which in turn frequency modulates the voltage controlled oscillator. The center frequency at which the voltage controlled oscillator oscillates is set in combination with a tank circuit. These tank circuits consist of a capacitor, an inductance and a second varactor across which a voltage is applied to set the center frequency. Thus, the prior art voltage controlled oscillators utilize two varactors, an expensive discrete element.
At radio frequencies, the intended environment for this type of voltage controlled oscillator, the construction of the circuit and the oscillator transistor introduces many parasitic elements which influence the interaction of the tank circuit and the oscillator transistor. In general, the influence of these parasitic elements is to degrade the performance of the voltage controlled oscillator from the optimum obtainable in an ideal circuit.
Designers have typically focused upon removing, or at least alleviating, the effects of these parasitic elements. One such parasitic element is the inherent base to collector capacitance of a junction transistor. Many circuit configurations are known in the prior art to minimize the effect of this particular parasitic element.
It is an object of the present invention not to minimize the effect of the inherent junction capacitance of the oscillator transistor but to actually utilize it in the function of the oscillator.
It is a further object of the present invention to reduce manufacturing cost by removing the varactor used for frequency modulating the voltage controlled oscillator.
It is a further object of the present invention to isolate the transmission signal from the modulation signal so as to provide for a more manageable, controllable design.
This and other objects will become apparent in light of the attached specification and drawings.